Good day.Please advise whether i have any legal...

Good day.Please advise whether i have any legal obligation to sell my vehicle (which is still in the banks name) to a potential buyer with a mutual letter writing only on which we have both signed that i accepted an initial deposit as down payment AED 4,000 towards to full amount . But now i would like to change my mind to sell my car. But the person doesnt want to cooperate instead asking me AED 8,000. is the piece of paper hand writing that we both sign can be taken to legal action? Thank you and looking forward for your response